1. Field of the Invention
The present invention relates to medical imaging of body cavities, and more particularly concerns the use of magnetic resonance to obtain such images.
2. Description of Related Art
Imaging of cavities is closely related to the imaging of the interior volumes of blood vessels.
Angiography, or the imaging of vascular structures, is very useful in diagnostic and therapeutic medical procedures. MR angiography is performed with a variety of methods, all of which rely on one of two basic phenomena. The first phenomenon arises from changes in longitudinal spin magnetization as blood moves from one region of the patient to another. Methods that make use of this phenomenon have become known as "in-flow" or "time-of-flight" methods. A commonly used time-of-flight method is three-dimensional time-of-flight angiography. With this method, a region of interest is imaged with a relatively short repetition time, TR, and a relatively strong excitation radio-frequency (RF) pulse. This causes the MR spins within the field-of-view to become saturated and give weak MR response signals. Blood flowing into the field-of-view, however, enters in a fully relaxed state. Consequently, this blood gives a relatively strong MR response signal, until it too becomes saturated. Because of the nature of blood vessel detection with time-of-flight methods, the stationary tissue surrounding the vessel cannot be completely suppressed. In addition, slowly moving blood, and blood that has been in the imaged volume for too long, becomes saturated and is poorly imaged.
A second type of MR angiography is based on the induction of phase shifts in transverse spin magnetization. These phase shifts are directly proportional to velocity and are induced by flow-encoding magnetic field gradient pulses. Phase-sensitive MR angiography methods exploit these phase shifts to create images in which the pixel intensity is a function of blood velocity. While phase-sensitive MR angiography can easily detect slow flow in complicated vessel geometries, it will also detect any moving tissue within the field-of-view. Consequently, phase-sensitive MR angiograms of anatomy such as the heart have artifacts arising from the moving muscle and from the moving pools of blood.
Recently, new MR methods for imaging cavities in the body have been disclosed in "MRI Using Hyperpolarized Gas" by A. Johnson et al. p. 392, Proc. of the Soc. Magn. Resn., Third Scientific Meeting and Exhibition, Nice, France Aug. 19-25, 1995 Vol. 1. These methods employ a noble gas such as xenon or helium which is polarized by interactions with optically pumped rubidium. This method requires a laser and related apparatus. Also, the method requires that the rubidium be removed with a high degree of efficiency since rubidium is toxic. Noble gases are known to produce anesthetic effects, and can, in sufficient concentration, be considered to be toxic.
Currently, there is a need for a system for obtaining high quality MR imaging of a selected cavity within the body without the risks of exposure to ionizing radiation and X-ray opaque contrast injections and without use of materials hazardous to humans and animals.